Not applicable
1. Field of the Invention
The invention disclosed broadly relates to the field of accelerometers and more particularly to permanent magnet accelerometers for sensing rotational or angular acceleration.
2. Description of the Related Art
Rotary accelerometers have been in existence for years. Rotary accelerometers may be used to provide closed loop motion control of a load through the use of feedback techniques. The acceleration signal from an accelerometer may be used to electronically simulate larger, smaller or varying system inertia. One example of a DC (Direct Current) excited rotary accelerometer can be found in U.S. Pat. No. 2,090,521, entitled xe2x80x9cAccelerometerxe2x80x9d, by inventor Robert Serrell, and issued on Aug. 17, 1937. (Hereinafter this patent is referred to as Serrell.) This accelerometer makes use of the phenomenon of armature reaction in a rotating dynamo-electric apparatus to provide an indication of the magnitude of the acceleration. The magnitude of the magnetic field produced by currents induced in the armature of a generator varies with acceleration of the rotating member.
Another accelerometer is disclosed in U.S. Pat. No. 3,178,641 entitled xe2x80x9cDrag Cup Tachometer Accelerometerxe2x80x9d by inventor J. H. Varterasian, issued on Apr. 13, 1965. (Hereinafter this patent is referred to as Varterasian.) This is a DC excited accelerometer that provides a conductive drag cup of a tachometer generator with alternating current and direct current excitation. The device processes an alternating current and a direct current output signals which are a function of the drag cup speed and acceleration respectfully.
Another rotary accelerometer is disclosed is U.S. Pat. No. 4,507,607, entitled xe2x80x9cAngular Accelerometerxe2x80x9d by Inventor William R. Caputo, issued on Mar. 26, 1985. (Hereinafter this patent is referred to as Caputo.) Disclosed is an angular accelerometer with DC excitation which generates a direct usable signal and stationary coil proportional to the instantaneous acceleration rate of a rotary element.
Yet another rotary accelerometer is disclosed in U.S. Pat. No. 3,555,326 entitled xe2x80x9cAccelerometer for a Vehicular Anti-Skid System with Sheet Loaderxe2x80x9d by Inventors Abootaleb Talebi et al., issued Jan. 12, 1971. (Herein after this patent is referred to as Talebi.) This permanent rotary accelerometer has a solid cylindrical ferromagnetic rotor rotationally coupled to a vehicle wheel and coated with conductive non-ferromagnetic metal sheet.
Although all these rotary accelerometers are useful they are not without their shortcomings. One shortcoming of other acceleration processes is the use of position or velocity signals to derive acceleration signal. The double differentiation of a position signal or the single differentiation of a velocity signal is accomplished using analog and digital differentiation circuitry. This differentiation process adds electrical noise to the output signal. This noise is undesirable for high performance systems. Accordingly, a need exists for an accelerometer that eliminates the differentiation circuitry and provides an acceleration output signal directly.
Continuing further, several of the patents by Serrell, Varterasian and Caputo referenced above, use DC excitation. One shortcoming of the use of DC excited accelerometers is that the output gain signal for the acceleration is dependent on the excitation signal for the direct current excitation. The dependence on the excitation signal can lead to a wide drift in accelerometer output signal. Therefore, a need exists to provide an accelerometer whose output voltage is more stable and does not fluctuate when the input voltage shifts.
Another shortcoming of the use of DC excited accelerometers is that a power source is required to produce the DC voltage along with the companion filtering circuitry and often times a rectifier to condition the power source. These additional components add cost and weight, and besides adding cost and adding weight, also increase the size of the accelerometers. Large accelerometer configurations are difficult to integrate on a motor or other rotating machine. Many times, designer engineers make use of mechanical couplings to integrate motors into specific applications. However, these couplings add weight and space. In addition, these couplings contribute to windup and electrical noise in the accelerometer output signal. Accordingly, a need exists for a compact configuration of an accelerometer which may be integrated directly into an electric motor or other similar rotating machine.
Another shortcoming with the use of DC excited accelerometers is that in regards to the output, ripples, fluctuations or any noise present in a DC excitation source are translated to the accelerometer output signal. This produces an undesirable signal-to-noise ratio on the output. To eliminate this undesirable noise, the output signal requires filtering which adds lag between the rotation of the shaft being measured and the output of the signal being produced. Accordingly, a need exists for a rotary accelerometer that provides a direct output signal and does not require excessive filtering or demodulation.
Another shortcoming of the use of DC excited accelerometers is that output power level or gain is lower than permanent magnet excitations. Stated differently, the output voltage per unit of acceleration is much lower within a given frame size. Accordingly, a need exists to provide an accelerometer that has high output voltage levels.
Turning now to the permanent magnet accelerometers available today, such as the permanent magnet accelerometer disclosed by the Talebi patent, there are several shortcomings. One shortcoming of Talebi as shown in FIG. 2 is that pick up coil 6A and 6B are separate from magnetic shells 4 and 5 of FIG. 2. These separate pieces reduce the amount of sensitivity that can be measured with this device. Accordingly, a need exists to provide a permanent magnet accelerometer with high acceleration sensitivity.
Another shortcoming of the permanent magnet accelerometer disclosed by the Talebi patent is that the rotor uses a coating that is galvanically (electroplating) applied. The uniformity of coatings and platings are difficult to maintain during the manufacturing processes. Moreover, environmental conditions such as shock and vibrations reduce the effectiveness of the plating. Accordingly, a need exists for an accelerometer that overcomes this shortcoming and is able to perform in harsh environments.
Still another shortcoming of the permanent magnet accelerometer disclosed by Talebi is that the stator winding core pieces 7, 8 and 9 of FIG. 1 and FIG. 2 are constructed from distinct pieces of ferromagnetic material. Stated differently, the stator winding core pieces 7, 8, and 9 are fabricated from at least three different pieces. The use of separate stator winding core pieces does not provide maximum sensitivity of pickup coils 6a and 6b due to the increase in magnetic reluctance. Moreover, for the application of accelerometers in a harsh environments such as aerospace applications, the exposure to wide temperature fluctuations, vibrations, and shock and high g-forces is not structurally sound. Accordingly, a need exists for an accelerometer that has very high sensitivity even in demanding applications in harsh environments.
Briefly, in accordance with the present invention, a rotary accelerometer is used to complement closed loop control systems such as brushless D.C. motors, stepper motors, A.C. motors, hydraulic motors, pneumatic motors, or any other device which is controlled through rotary motion. The rotary accelerometer provides a direct output of the acceleration signal and can be integrally mounted to a motor or other rotating machine. The four components whose configuration changes in the various embodiments are as follows: (i) nonrotating permanent magnets; (ii) a nonrotating stator core; (iii) a rotating electrically conductive cup; and (iv) a rotating or nonrotating (depending on the configuration) magnetically permeable return path. Five different embodiments are described for various configurations of these four components.
In one embodiment, a permanent magnet rotary accelerometer comprising: a cylindrical magnetic return path formed out of magnetic permeable material; an electrically conductive cup with an axis for rotation there around, the cup surrounding the magnetic return path; one or more sets of permanent magnets having opposite magnetic polarity, surrounding the cup, for a producing magnetic field through the cup into the magnetic return path, thereby inducing eddy currents inside the cup when rotating in the magnetic field; a stator core formed out of magnetic permeable material surrounding the cup and nonrotatable relative thereto, the stator core comprising at least two pick up members and a stator core return path therein, wherein the pick up members and the stator core return path are formed from at least one continuous piece of magnetic permeable material; and one or more pickup coils disposed on the pickup members of the stator core for sensing changes in a secondary magnetic filed generated by the eddy currents in the cup.